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Shine on, shine on LED, for better displays.

The science fiction image of a gigantic, flat television screen hanging on a living room wall has loomed large in Hollywood movies. It's still just a pipe dream, but researchers in Britain, working on a luminescent plastic diode, have now taken the first few steps toward making that vision tangible.

Physicist Neil C. Greenham and chemist Stephen C. Moratti, both at the University of Cambridge in England, have produced a light-generating polymer with high enough efficiency to be useful as a light-emitting diode (LED). Conventional semiconductor LEDs, make mostly of inorganic materials, are now widely used in the displays of electronic equipment, from digital watches to control panels in autos, aircraft, and computers. The new plastic light sources reportedly work as well as most LEDs now in use and may soon surpass them in efficiency. Though still in the research phase, the polymer LED may eventually permit enhanced color displays large, flat-screened monitors.

The researchers report their results in the Oct. 14 NATURE.

Three years ago, physicist Richard H. Friend and chemist Andrew B. Holmes, both at Cambridge and coauthors of the current report, and their colleagues first reported polymer electroluminescence, but with impractically low efficiencies.

By March 1992, using a material called poly (p-phenylene vinylene), or PPV, Friend and Holmes had made the efficiency 30 times greater, yet it was still below that of existing LED technologies (SN: 3/14/92, p.164).

Now they report efficiencies above those of current LEDs from a new family of materials called poly(cyanoterephthalylidene)s. "By efficiency, we mean the number of photons emitted versus electrons injected," says Friend. "Crudely put," adds Holmes, "for every 100 electrons you inject into the material, 4 photons pop out."

Thus, the new polymer has an efficiency of 4 percent, with a theoretical limit of 25 percent, says Holmes. In comparison, typical inorganic LEDs operate at efficiencies of 1 to 2 percent, he says. (A tungsten lightbulb reaches 10 percent.) "In terms of efficiency, we're now very good," says Friend. "We've come up by a factor of 400 since 1990. This new cyano polymer is now competitive with most other LED technologies for display applications."

The "semiconducting conjugated polymer" emits light when positive and negative charges, supplied by electrodes on opposite sides of he thin plastic film, meet on the same bit of polymer chain. "Their meeting causes an excited state in the chain, which emits a photon in the visible wavelength," Holmes explains.

The key to raising the new polymer's efficiency was improving its ability to accept negative charges, Holmes says. The researchers sandwiched PPV in between layers of the new cyano polymer to help charge transport, and they replaced troublesome calcium cathodes with simpler aluminum ones, further improving the film's operation.

By altering the polymer's physical makeup slightly, chemists can change the colors to emits, "almost as if it were a dye," says Holmes. "We've got a way to tune the wavelength, or color, of the light emitted by altering the shapes and positions of the polymer's building blocks. We should be able to produce all of the primary colors and improve color purity."

Lest onlookers envision a road to the future paved in glowing plastic, both Holmes and Friend caution that there is still a long way to go, with no applications readily at hand. "But," adds Holmes, "there really are enormous possibilities here."

Examples include light-emitting clothing for police or rescue workers, glowing tools, radiating vehicles -- perhaps even the proverbial wall-size flat television to hang like a painting in the parlor. "Flat-screened TVs and better computer displays are what we're all after here," Holmes confesses, "a sort of LED Holy Grail."
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Title Annotation:poly(cycnoterephthalylidene)s used to make light-emitting diode
Author:Lipkin, Richard
Publication:Science News
Article Type:Brief Article
Date:Oct 16, 1993
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